Recently electric vehicles, which deliver less environmental burden than conventional vehicles using fossil fuel, are under development. If the electric vehicle is required to perform the same performance as the conventional vehicle, a high-cost secondary battery need be used and hence the secondary battery impedes wide application of electric vehicles.
It is proposed to provide a system, which promotes electric vehicles by effectively using a secondary battery of an electric vehicle as an electric power source for home use (for example, JP-A-2002-315193).
This system includes a switching device, which connects either one of a commercial AC power supply system and a secondary battery to a load at a home and disconnects the other and the home load. The switching device performs either one of a first state and a second state of connections. In the first state, the electric power is supplied from the commercial AC power supply system to the home load. In the second state, the electric power is supplied from the secondary battery to the home load.
For example, as shown in FIG. 24, a switching part 10A and a control part 60A, which controls the switching part 10A, are provided. The switching part 10A is formed of a switch 11a and a switch 12a. The switch 11a is a power supply system side switch provided between a commercial AC power supply system 5 and a load 3. The switch 12b is a secondary battery side switch provided between a secondary battery 2a mounted on an electric vehicle 2, which includes a DC-AC conversion part 2b, and the load 3.
For example, in a case that the power supply is switched from the first state, in which the load 3 is connected to the power supply system 5, to the second state, in which the load 3 is connected to the secondary battery 2a side, it is necessary for the control part 60A to turn off the switch 11a and then turn on the switch 12a after confirming that the switch 11a has actually been turned off so that the power supply system side and the secondary battery side are not connected to the load 3 at the same time. In addition, it is necessary to switch over from the first state to the second state in a short time so that the momentary shutdown of the power supply to the load 3 is avoided.
It is therefore proposed to confirm that the switch 11a actually turned off by checking whether the switch 11a actually turned off based on a load current flowing in a load line 4 between the switches 11a, 12a and the load 3 and a voltage waveform of an inter-phase AC voltage on the load line 4. However, the AC voltage waveform and the load current waveform, which appear immediately after the switch 11a is turned off, vary in accordance with the load 3. For this reason, it is likely that the switch 11a is erroneously determined to have not turned off although actually turned off. It is also likely that the switch 11a is erroneously determined to have turned off although not actually turned off in a case that the switch 11a has an on-failure. The on-failure is a failure that the switch 11a does not switch over from an on-state to an off-state, thus persistently remaining in the on-state.